| ||For the modified FEV strain in Fallout 2, see FEV Curling-13.|
Vial inserted in the Auxiliary Filtration Input
The modified FEV is a quest item in Fallout 3. It is given to the Lone Wanderer by John Henry Eden in Raven Rock and can be inserted in the Auxiliary Filtration Input in Jefferson Memorial's Rotunda, contaminating the purifier's filter system and subsequently, the Capital Wasteland's waters during Take it Back!.
The modified FEV is a heavily modified sub-strain of the Forced Evolutionary Virus that was developed by John Henry Eden. It is said to be based on or related to a progenitor, the FEV Curling-13 sub-strain the Enclave's Chemical Corps had originally synthesized and tested in 2242.
Some changes include its intended design of being compatibly inserted into a water purification system's schematics, such as the one found at the Jefferson Memorial. Also, highly unlike the FEV Curling-13, it does not target those that are pure humans.
According to President Eden, it will indiscriminately target and exterminate super mutants, ghouls, any and all other mutated creatures, and generally anything that has been at all effected by Wasteland mutation upon ingestion. This would include any wastelander that had been originally born in the Wastelands and/or heavily exposed to its background radiation and other mutational hazards, or hosts any other sort of mutational side effect at all as a result.
Theoretically, the only people who would prove immune to infection are dangerous radiation level/mutation free pure humans, those that are born inside Vault-Tec vaults or those born inside other similarly sealed environments and as a result, uneffected by Wasteland mutation. This is proven to be accurate in one of the ending slides if the player completes Project Impurity. 
In the add-on Broken Steel, if the player chose to contaminate the Purifier with the modified FEV, the medical facility of nearly every major settlement in the Capital Wasteland (except Tenpenny Tower and Paradise Falls) has at least two or three newly arrived sick patients lying on the beds, dying from modified FEV infection.
The patients hold their stomachs and moan that "it burns on the inside." If/when the player speaks with one of the doctors in a major settlement, they state that the're fairly certain that 'the condition' is not contagious to their knowledge. 
Doctor Preston does state however that they are burning the corpses of the infected dead just to be safe, out of fearful precaution that drinking the water might indeed transmit or otherwise result in a contagious plague of some kind developing as a result. 
- With Broken Steel and if the player drinks three successive bottles of Aqua Pura after having completed Project Impurity and inserting the modified FEV, they will die.
- It's not possible to leave the room where President Eden is located unless the player has picked up and retrieved the modified FEV from Eden's console.
- This item uses the world model of an empty Nuka-Cola bottle as a placeholder, likely because, as a quest item, it can never be dropped normally.
- It can be given to Elder Lyons for good Karma if spoken to after escaping Raven Rock. With Broken Steel, the modified FEV will be removed from the player's inventory if they activated Project Purity but did not give the modified FEV virus to Elder Lyons beforehand.